Imagine that it’s a Sunday morning, and you are pretty bored and want to do something relaxing and a little bit of fun. Imagine that you talked about LEDs and Arduinos with a colleague two days before, and so you have checked your electronics storage and know perfectly how many LEDs you still have (too many, I can assure you). Why not do something fun with them? For example, LEDs that play along a MIDI track 🙂
So, what I did was:
- find out a nice MIDI track, a not too complex one if possible; I decided for “Twenty One” by Jessica Curry, from the “Dear Esther” soundtrack: I love that theme, and it’s basically a single instrument (a piano). I already had a MIDI version of it, which I simplified a little bit (I basically removed the counter-theme)
- find out how many 220 Ω resistors you have (turns out: about 40, that go along perfectly with the 36+ red LEDs that I had)
- find out how many shift registers (the famous 595 IC) you have: not so many, unfortunately. I only had 3, so I can pilot up to 24 LEDs, or 2 octaves. I would have preferred 3 octaves…
I put together the circuit (I tried to resemble a piano keyboard, with two rows of “keys”; if I had more LEDs of a different color, the result would have been better. Also, I didn’t want to solder anything, so the positioning on the breadboard cannot be as precise as one would like. I also needed two more LEDs, that I had to switch on and off from the Arduino side without the 595) and the code: it uses the MIDI library to receive the “note on/off” messages from the serial line (you can see the RX LED switching on the Arduino in the video), and it switches the LEDs according to the pitches that it receives. Since I only had 2 octaves, I had to transpose the theme one octave down, and since the MIDI file had 3 tracks but I had just one line of LEDs, I had to prioritize the tracks: basically, if the same note is played by two tracks, the track with the highest priority would decide when to stop the note. I achieved this by reordering the tracks in the MIDI file, so that the ones with a highest channel number have a highest priority.
The last step from the Arduino point of view was install the dear old Hiduino firmware: basically it allows the Arduino to be recognized as a MIDI device, instead of a serial device, so that it can communicate with multimedia apps on the computer side. Of course, once you rewrite the firmware, you need to upload the sketches from the ISP header.
Finally, I fired up Jack (the low latency audio daemon), Rosegarden (to reproduce the MIDI file) and KMidiMon (to monitor the MIDI events), I connected the Arduino to the computer, routed the MIDI lines to all the “clients”, and voilà!
P.S.: nevermind the names of some of the variables in the code: they come from previous experiments with the 595s, and I haven’t changed them due to laziness…
Syvo3 
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